The fastest way to waste a founder's time is to spend the first twenty minutes having them explain what they already sent you. Preparation is a basic form of respect, and it's what separates a useful mentor from a well-meaning one.
Read the materials. Look at the product. Check the website, the deck, the numbers, the question they asked. Arrive with a point of view and two or three specific questions that only someone who did the homework could ask.
Preparation also means knowing what the founder needs from this particular conversation. If it isn't clear, ask before you meet. A focused thirty minutes beats a wandering hour.